1. Technical Field of the Invention
The present invention relates to a gas sensor, and more specifically relates to a gas sensor which is provided facing a predetermined flow path of a gas and includes a detection element for receiving a predetermined signal to vibrate and detecting means which transmits a vibration wave generated by the vibration of the detection element in a flow path direction to detect characteristics of a gas.
2. Description of the Related Art
Conventionally, a gas sensor is known which detects, for example, a concentration, a temperature, or a humidity of a specific component as characteristics of a gas present in a flow path, using a detection element. In such a gas sensor, a signal from the detection element is electrically processed and output as an electric signal corresponding to the characteristics of the gas. As an example of the gas sensor, a gas concentration sensor which is provided in transportation equipment mounted with an internal combustion engine such as an automobile and which detects a concentration of gasoline, gas oil, or the like utilizing a change in propagation speed of a vibration wave of sound will be discussed.
Such a gas sensor is arranged, for example, in a passage for purging gasoline from a canister mounted on the automobile to an inlet pipe of the internal combustion engine. The gas concentration sensor includes a flow path of a predetermined volume through which a gas containing gasoline vapor in the above-described passage flows and a detection element which is provided facing this flow path and detects a gas concentration. The gas concentration sensor vibrates the detection element in detecting the gas concentration and transmits a vibration wave (e.g., ultrasonic wave) generated by this vibration in a flow path direction. Such a vibration wave, which is transmitted in the flow path direction of the gas from the detection element by the detection element vibrating for detection of the gas concentration, is hereinafter referred to as a vibration wave for detection. The speed of the vibration wave passing through the flow path changes according to a concentration of gasoline vapor existing in the flow path. The gas concentration sensor detects the speed of the vibration wave for detection passing through a flow path of a fixed flow path length with a receiver for receiving the vibration wave for detection, finds the concentration of the gasoline vapor as a result of this detection, and outputs it.
In such a conventional gas sensor, the detection element for transmitting the vibration wave for detection is arranged in a housing which is formed of resin or the like having high heat resistance (e.g., see JP-A-2000-206099). In addition, since it is necessary to keep a position of the detection element constant in order to accurately detect the speed of the vibration wave for detection passing through the gas flow path, a filled layer is formed by filling a filler such as urethane in the housing, in which the detection element is arranged, to seal the detection element and regulate a positional movement of the detection element.
3. Problems Solved by the Invention
However, in such a gas sensor for detecting a gas concentration utilizing a vibration wave from a detection element, reverberation may occur in a filled layer in which the detection element is embedded following vibration of the detection element, and as the detection element is affected by this reverberation, it is likely that characteristics of a gas such as a gas concentration cannot be detected accurately.
That is, in the case in which a detection element is used both for transmission and reception, it is likely that, if a reverberation in the filled layer lasts a long time, a vibration wave emitted in a direction different from the transmission direction of the vibration wave for detection by vibration of the detection element affected by this reverberation (hereinafter referred to as noise vibration wave) interferes with the vibration wave for detection, and accurate detection of a gas concentration based upon the vibration wave for detection cannot be performed. In addition, in the case in which the element for transmission and the element for reception are provided separately, it is also likely that, if a reverberation in the filled layer lasts a long time, after a vibration wave for detection is transmitted to a gas flow path, a noise vibration wave is transmitted to the gas flow path from the detection element affected by the reverberation, and accuracy of detection of a gas concentration based upon the vibration wave for detection falls.
The present inventors observed that such reverberation continues over a longer period at higher temperatures. Therefore, in an environment in which temperature tends to increase such as in the vicinity of a passage for purging gasoline or a gas flow path in the gas sensor, reverberation may increase to make accurate detection of gas concentration difficult.
In addition, in a conventional gas sensor, it is likely that, in the case in which temperature increases in the gas sensor, the filler in the housing thermally expands and a position of the detection element moves in the housing following this thermal expansion. In the case in which such positional movement occurs, since a flow path length of the gas flow path changes and the time for the vibration wave for detection passing through the gas flow path changes following the change in the flow path length, it is likely that accuracy of detection of a gas concentration based upon the vibration wave for detection falls.